1. Field of the Invention
The present invention relates to a capacitance detecting circuit and detecting method for detecting a small capacitance, and to a fingerprint sensor employing the same.
2. Description of the Related Art
Hitherto, as a fingerprint sensor, which is considered as most promising among techniques of biometrics, a pressure-sensitive capacitance sensor of a type in which column lines and row lines are formed at a predetermined pitch on surfaces of two films, respectively, the films being disposed opposing each other via an insulating film or the like with a predetermined gap provided therebetween, has been developed. In the pressure-sensitive capacitance sensor, when a finger is placed, the shapes of the films change according to the ridges and valleys of the fingerprint, and the gap between the column lines and the row lines change according to the change in the shapes of the films. Thus, the form of the fingerprint is detected as small changes in capacitances at the intersections of the column lines and the row lines. In the pressure-sensitive capacitance sensor described above, a small change in capacitance less than several hundred fF (femtofarads) must be detected at each of the intersections of the matrix of the column lines and the row lines. Thus, regarding a capacitance detecting circuit of the capacitor sensor, a demand for high sensitivity exists since the change in capacitance is small, and a demand exists for insusceptibility to noise that is transmitted from a human body or noise that occurs in circuitry and for insusceptibility to crosstalk between column lines or between row lines.
In a capacitance detecting circuit of the type described above, adjacent detecting lines of capacitors (sensor elements) are respectively connected to a non-inverting input terminal and an inverting input terminal of a differential amplifier circuit, and the difference between voltage values of detection signals of the detecting lines is amplified, so that noise superposed on the detection signals is cancelled to a certain extent, suppressing the effect of the noise. Such a circuit is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 11-118415 paragraphs [0011] to [0064] and FIG. 2).
In the capacitance detecting circuit, at a plurality of small capacitors (sensor elements) disposed on a row-by-row basis, the small capacitors are charged and discharged by a charge amp circuit, and differential detection is carried out for adjacent small capacitances on each row.
At this time, on each row of small capacitors, the combination of capacitors for differential detection is sequentially changed, allowing two-dimensional detection of capacitances.
The measurement techniques for differential detection can be applied to a capacitance detecting circuit of a sensor in which column lines and row lines cross each other and sensor elements are disposed in a matrix shape. Measurement signals of adjacent row lines are input to a non-inverting input terminal and an inverting input terminal of a differential amplifier to detect a differential component of charging or discharging associated with a driving pulse of a column line, so that reduction in noise due to crosstalk between row lines is expected.
However, in the conventional capacitance detecting circuit described above, the arrangement is such that a difference in voltage or current based on capacitances of adjacent sensor elements is detected.
Since the conventional capacitance detecting circuit lacks a reference for a subject of measurement, the level of a detection signal is determined by capacitances of adjacent detection lines, so that uniform detection is inhibited even in a single row. Thus, the mutual effect of the detecting lines is not fully eliminated, causing degradation in the resolution of detecting capacitance by the sensor elements.
Furthermore, in the conventional capacitance detecting circuit, although it is possible to decrease to a certain extent the effect of feed through of a sensor unit and switches respectively provided for the rows for introducing detection signals in a differential amplifier circuit, it is difficult to fully eliminate the effect of layout of lines or variation in stray resistance and stray capacitance of input selector.
Thus, the conventional capacitance detecting circuit has the drawback that it is not possible to obtain uniformly precise measurement results over the entire surface of a two-dimensional sensor such as an area sensor.